Drilling, used to create long boreholes, typically involves a plurality of elongate drill string rods coupled together end-to-end by interconnected male and female threaded ends. As will be appreciated, the threads of both the male and female ends are subject to stress concentrations which results in material fatigue and a reduction in the service life of the drill string component. In general, the stress concentrations tended to be highest at one or both of the axial ends of the thread particularly as a result of bending moments imparted to the coupling due to large lateral shifts of the drill string during boring and potentially during retrieval preparations. Attempts to improve the threaded joints are described in WO 2007/133145 and WO 2008/150207.
EP 1066448 describes a threaded coupling for percussive drilling in which a cylindrical external thread has at least one thread end in which the thread depth decreases in an attempt to reduce the likelihood of damage to the thread. However, existing threaded joints are still susceptible to stress concentrations and fatigue during use and there exists a need for a threaded coupling that offers enhanced strength and durability over existing configurations.